This disclosure relates to unstable laser resonators, and more particularly to unstable laser disk resonators.
The above cross-referenced patent application, which is fully incorporated herein by reference, discloses an unstable laser disk resonator capable of providing a high power, near diffraction limited laser signal that is suitable for precise laser applications. The high power signal is achieved by disposing a plurality of 1:1 laser disk imaging systems in a cascaded arrangement along the optical axis of the resonator cavity; between the cavity's Primary and Secondary (feedback) mirrors. Each of the 1:1 imaging systems includes comprise two optical elements, such as a laser disk and a reflecting mirror, which is also referred to as a relay mirror, or two laser disks.
The optical elements of each 1:1 imaging system are disposed in a confocal relationship on opposite sides of a virtual symmetry axis that runs through their common focal point. The elements are also positioned in a mutually oblique relationship such as to provide the cascaded imaging systems in a “W” configuration along the symmetry axis. This allows for full self imaging of each laser disk output signal onto each adjoining laser disk to achieve a high power laser output.
The referenced unstable imaging laser resonator may be embodied as either a positive branch or a negative branch resonator to provide either a positive branch imaging resonator (PBIR) or a negative branch imaging resonator (NBIR). The NBIR has the advantage of being less alignment sensitive and it naturally compensates for system astigmatism due to its odd number of lasing signal foci. This allows greater flexibility in interchanging the positions of the laser disk and the reflecting mirror within the imaging system, and it also makes it possible to replace the reflecting mirror with a laser disk so that both optical elements of the 1:1 imaging system may be laser disks.
The NBIR embodiment of the referenced unstable imaging laser resonator employs a standard end-mirror placement in which the Primary and Secondary end mirrors are in direct optical communication with opposite end ones of the plurality of cascaded 1:1 imaging systems. The Primary mirror receives the return path laser signal directly from the 1:1 imaging systems and the Secondary mirror also receives the forward path laser signal directly from the 1:1 imaging systems. The Secondary mirror couples a portion of the forward path laser signal out of the cavity and reflects the remainder to the imaging systems to establish the feedback propagation of the resonant laser signal. The imaging system laser disks have an active medium, such as Yb:YAG (ytterbium-doped yttrium aluminum garnet), which is excited by a pumping light from a diode laser array. The pumped laser light restores the energy to the disk medium that is extracted from the lasing light. This allows the laser disk to provide a greater than unity gain at the laser wavelength, and function as an active mirror.
Although the prior art NBIR is less alignment sensitive, and naturally compensates for system astigmatism, all high-power laser resonators have residual aberrations that can occur with random physical displacement of the NBIR optical elements, or from variations in the pumped laser excitation of the laser disk medium. Prior art methods for correcting and/or compensating for these aberrations include the use of closed-loop multiple-actuator deformable mirrors. These corrective systems, however, are both complex and expensive. In addition, there are no current actuated mirror mount designs that are capable of correcting for odd order aberrations, such as comatic aberrations, or “coma”. It is desirable, therefore, to find a method or apparatus which can suppress all aberrations, including such odd order aberrations.